1. Field of the Invention
The invention relates to a self-testing proximity switch having an oscillator; a demodulator; a trigger stage; a clocked test pulse source feeding the oscillator with test pulses having positive and negative amplitudes sufficiently high to trigger the trigger stage from either one of its switching states to the other for the duration of a single pulse in order to simulate a damping reduction or damping of the oscillator; and a test circuit responding to the level changes in the output signal of the trigger stage occurring in response to the test pulses, and serving to thereby generate a test output signal.
2. Description of the Related Art
A self-testing proximity switch of the kind having a selectively damped oscillator has been described in European laid-open patent application No. 0 035 225. The proper functioning of this proximity switch is tested by determining whether the oscillator is damped by a test pulse or whether the oscillation amplitude of the oscillator is reduced to zero by the test pulse, to simulate the approach of an element to be detected toward the active surface of the proximity switch.
This type of functional test fails to detect faults which result in the output signal of the proximity switch indicating a damped state of the oscillator even though the oscillator is not actually damped by an element brought sufficiently close to the active surface of the proximity switch. This disadvantage is eliminated in the proximity switch disclosed in German laid-open patent application No. 3 007 929 and further described in Hermle et al. U.S. Pat. No. 4,433,309, wherein oscillator damping reduction is also used for test purposes. In these proximity switches, the internal operating parameters of the oscillator are influenced to cause a change in the oscillation of the oscillator which corresponds to a certain change in position of an element, for example, a trip cam, that influences the resonant circuit externally. In other words, an external damping or damping reduction is simulated by internally influencing the internal operating parameters. These kind of proximity switches use so-called breakaway oscillators, wherein the oscillation breaks away (i.e., terminates) in the event of external or internal damping.